It is well known that the variations of the support force exerted by a main rotor of a rotorcraft are obtained by varying simultaneously and in the same sense the incidence of all of the blades of this rotor (overall or collective pitch control) and that the steering of a rotorcraft, the shaft of each support rotor of which is maintained in a fixed position on the structure of the rotor craft, is effected by varying cyclically the pitch of the baldes (cyclic pitch control).
With the object of transmitting control commands for the overall pitch and the cyclic pitch to the blades of a rotor it has already been proposed to use apparatus comprising two annular plates assembled together via at least one ball bearing and mounted concentrically around the shaft of the rotor and of which one is a non-turning plate fixed in terms of rotation by at least one lower link member fixed to at least one housing rigidly connected to the structure of the rotorcraft and mounted on the one hand so that it is displaceable in the direction along the shaft of the rotor and on the other hand oscillating relative to the shaft of the rotor via the action of an articulation mechanism, while the other is a turning plate brought to rotation with the shaft of the rotor by at least one upper link member fixed to the shaft and/or to the hub of the rotor and connected to a control lever for the pitch of each of the blades of the rotor via a pitch rod, the non-turning plate being acted on by control members for the general pitch which control its displacement along the shaft as well as by cyclic pitch control elements, which effect its oscillations relative to the shaft, in such a fashion that the turning plate, which follows the movements of the non-turning plate, transmits these movements to pitch control levers via patch rods.
Thus control of the collective pitch is obtained by axial translation of the assembly of the plates, effected by axial sliding along the shaft of the rotor of the articulation mechanism, and control of the cyclic pitch is obtained by an inclination of the assembly of the plates relative to the axis of the shaft of the rotor thanks to the articulation mechanism.
From French Patent Specification No. 1049106 and its Certificate of Addition Number 63478 a device of the type stated is known in which the rotating plate is in an externally radial position relative to a single ball bearing which joins the two plates and which has the form of a star having as many branches as the rotor as blades and which is brought into rotation via a rotating caliper which forms the upper link element and is pivoted by its upper end on the hub. This star-shaped plate is mounted rotatably on the non-turning plate which is in a radially inner position and is articulated relative to the shaft via a universal joint in such a way as to enable it to be inclined in all directions in space under the action of the two "V" arms which constitute the cyclic pitch control elements. The control elements for the collective pitch and the lower link elements are formed by means of a common mechanism comprising a two armed fork pivotally mounted on the non-turning plate around a diametric axis of the shaft and a box on which there is pivotally mounted about an axis perpendicular to the axis of articulation of the arms of the fork with the object of providing a universal joint, the tail of the fork, this box being connected to a support fixed relative to the shaft box of the rotor by means of a pair of lateral rods and a triangular member itself articulated on the other hand on the support, and finally via a swivel joint to a short control rod for the collective pitch.
Likewise an analogous apparatus is known in which the articulation mechanism is a swivel joint on which the non-turning plate is mounted in oscillating fashion and which is itself mounted slidingly axially around a guide plate coaxially surrounding the shaft of the rotor and fixed to a casing integral with the structure of the rotorcraft, the upper link member being a caliper the upper end of which is integral with the shaft, the lower link member being likewise a caliper of which the lower end is integral with the casing and the control elements being servo controls.
These two approaches have the common disadvantage that, with the object of ensuring a radial positioning appropriate for the plates, it is necessary to mount the constituent parts with very little play. These mechanisms necessitate good lubrication of pieces which rub on one another. However despite this precaution, wear can appear after a certain service period which increases the play and can prevent the good functioning of the control elements. In particular, in the second embodiment noted above, the precise adjustment between the cooperating bearing surfaces of the swivel joint and of the non-turning plate as well as between the swivel joint and the guide plate is the origin of substantial friction which generates wear in the sliding mounting of the swivel joint and between this latter and the plate which it carries in such fashion that the risks of play appearing as the origin of vibrations, or the risks of jamming of the controls are increased. It is therefore necessary to carry out frequent maintenance.
In addition it should be noted that the first known embodiment which has been set out above is relatively complex and requires a large number of components.
With the object of avoiding these disadvantages it has been proposed in French Pat. No. 2119828 that the turning plate should be an inner plate carried by the shaft of the rotor via a joint comprising an interior ring having the general form of a spherical equatorial zone which is integral with the shaft and an external ring of a generally internal cylindrical form which is fixed to the inner plate, these rings being distanced one from another and having a certain number of axial grooves regularly spread about them and facing one another which surround, in each pair of facing grooves, a ball, a cage located in the space between the rings keeping the assembly of the balls in position while the non-turning plate is an external plate having essentially the form of a cylindrical annulus carried via two ball bearings by a cylindrical turned down skirt of the interior plate and this latter is reinforced by a tubular torus extending radially towards the exterior and supporting the joints of the pitch rods.
However this known embodiment has two disadvantages in common with the two other embodiments of the state of the art since, in all three cases, the pieces of the articulation mechanism for the plate which is interior relative to the shaft are subjected to dynamic stresses when working and are highly stressed, and as a result of the annular structure of a substantial number of components, it is necessary in order to change even just one of these pieces to dismount the rotor hub and sometimes to dismount the shaft which, with certain mountings of the latter, likewise entrains the dismantling of the output drives of the main transmission box which ensures the setting in rotation of the rotor shaft. This has the result of increasing by a very considerable amount the length of time and the cost of maintenance work.